Scientific Forecaster Discussion
fxus65 kslc 211102
Area forecast discussion
National Weather Service Salt Lake City Utah
502 am MDT Mon may 21 2018
Synopsis...a drier southerly flow will set in today ahead of a
storm system that will create more unsettled weather Tuesday into
Wednesday. A ridge will build in for the end of the week.
Short term (through 12z thursday)...
extreme drought is occurring from Provo south to Cedar City and
to the southeast. Severe drought is occurring surrounding the
extreme drought, in areas south of Ogden and Duchesne to the
Arizona border. Drought conditions scale down to abnormally dry
along the Idaho and Wyoming border.
the pattern has re-amplified across the Pacific with a deep closed
low near 50n 160e, and another near 55n 150w. Each system is
juxtaposed with strong upper level jets.
Water vapor satellite shows a closed low over central California.
400-250 mdcars wind observations show a cyclonic 125-160kt jet
located south of the Aleutian Islands. This jet has carved out a
ridge over the eastern Pacific and a trough over the West Coast.
Local observations and trends...
00z kslc radiosonde observation shows steeper lapse rates and stronger flow compared
to 24 hours earlier. Precipitable water values range from
0.10"-0.20" southwest Utah to 0.50"-0.70" across the other valleys.
This reflects 0.15"-0.30" of lower (drier) precipitable water
values compared to 24 hours ago, with 0.10"-0.15" higher
(moistening) values across far western and northern Utah.
A 1003mb surface cyclone is located near Las Vegas with pressure
2-4mb lower than 24 hours ago across Utah.
Radar and infrared satellite indicate isolated weakening showers across
the region, though showers are holding together better near
24 hour trends: temperatures are generally little changed.
Dewpoint depressions are 10-20f higher (drier) across southern
convection this afternoon should favor northern and some central
areas, with no convection expected across the south during the
afternoon. Instability today appears to be higher based with
lower ncape values, suggesting that updrafts will not be as strong
as some were yesterday. Storm motion should generally be from the
south-southeast between 15-25kts.
As the closed low nears Las Vegas, height falls may support new
convective development across portions of southwest and south
central Utah with lingering convection further north this evening.
Arrival of dynamical forcing should also bring more expansive
cloud cover to the region Monday night, lasting through Tuesday
Best dynamical lift appears to be across southwest Utah this evening,
and eastern areas Tuesday as measured by the positioning of the
left exit region of the upper level jet, height falls and
diffluence aloft. The question for southwest Utah is whether
dynamical forcing is strong enough to overcome the dry airmass.
The answer is not yet definitive so left a mention of isolated to
scattered evening and overnight convective activity for all areas
except near Lake Powell.
Instability parameters improve across the region Tuesday thanks
to cooling aloft associated with the closed low. Expect to see
isolated convection closer to the Arizona border, increasing to
scattered coverage across northern and central areas Tuesday,
which may linger overnight across the north supported by better
instability and proximity to the decaying closed low across
northeastern Nevada. Storm motion should generally be from the south-southeast
between 15-25kts once again.
Wednesday appears to be a transition day. The aforementioned
closed low fills and opens, with the best proximity to shortwaves
associated with the opening system and height falls (early during
the day) across the north, which also coincides with the better
instability since drying will be occurring across southern and
central areas. Strong height rises occur across southern and
central Utah Wednesday with the weakening system progressing to
the northeast. Storm motion should generally be 10-20kts from the
A warming and drying trend is expected to continue today as
southerly winds become increasingly gusty ahead of the
aforementioned closed low. Cooler temperatures and moistening
lower levels are expected Tuesday due to expansive cloud cover
and cooling in the column driven by the arrival of the closed low.
Southern and some central areas should see a new warming and
drying trend begin Wednesday.
As for guidance, utilized the lowest relative humidity of the most reliable
guidance across southern Utah due to several periods of drying
and gusty southerly winds. Monday night through Tuesday night
leaned more on the higher relative humidity values based on the best guidance.
Also Tuesday utilized the coolest of all of the most reliable
guidance considering the likelihood of thick cloud cover and
cooling temperatures in the column.
Long term (after 12z thursday)...
as the remnants of the upper low continue to lift into the
northern plains Thursday, heights will build overhead yielding an
increasingly stable and drier environment across the area. Have
retained slight chance pops across the Utah/Idaho border and northern
mtns (largely diurnal) due to marginal maintenance of low level
moisture during the day, but as a whole the local pattern at that
time will be trending away from trough and towards a ridge for the
late week period.
The aforementioned ridge will build overhead through Friday in
response to the next closed low that will be gradually spinning
inland across central cali/western Great Basin heading into the
early weekend period. Increasing S-southeast mid level flow on the
downstream side of this low will maintain a dry advective flow
across the area. Temps expected to ramp up to some 10-15 degrees
above climo once again prior to trough influence.
In large global guidance has decent agreement regarding evolution
and track of the low thereafter as it interacts with the ridge in
place, further fills, whilst lifting NE across the area Sunday into
Monday. Slight timing differences exist between the operational
ec/GFS (00z), with the former a touch slower (similar with gefs
ensemble mean), but outcome of this should be negligible. As
current, the period of highest potential for precip potential exists
from later Sunday and into Monday, but details will fall out with
time. Continue to note a cooling trend this weekend into early next
week, with a slight chance of precip for that period.
southerly surface winds currently in place at the kslc
terminal are expected to switch to the northwest between 17-19z.
Scattered terrain based showers/storms are expected to develop mid
afternoon, with potential impact to the terminal greatest in the 22-
03z timeframe...with operational impacts tied to gusty and erratic
outflow winds (most likely southerly), and a slight chance of
erc values at RAWS sites are trending higher areawide, and range
from the 80th to 89th percentile southeast valleys, 50th to 69th
percentile southwest Utah and eastern valleys, and below the 50th
percentile further north and west.
Drying will occur across the south today, limiting convection to
the northern and central areas. Thunderstorms could ignite new
fires considering a low chance of wetting rain.
The warming and drying trend continues today as southerly winds
become increasingly gusty. Single digit relative humidity values are expected
across southern and southeast valleys this afternoon.
A broad storm system organizes across central California today,
impacting the region Monday night through Tuesday night, bringing
another round of showers and thunderstorms, along with short
lived cooling and moistening trends.
Short term/fire weather...10
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